# CVE-2026-6270: @fastify/middie vulnerable to middleware authentication bypass in child plugin scopes

> Attackers can bypass authentication in child plugin scopes by exploiting middleware inheritance flaws in @fastify/middie.

- **Severity:** Critical 9.1 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 16 Apr 2026
- **Updated:** 16 Apr 2026
- **Also known as:** CLSA-2026-1778201074, GHSA-72c6-fx6q-fr5w

## What is CVE-2026-6270?

CVE-2026-6270 is a critical authentication bypass vulnerability in @fastify/middie, published on 16 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

@fastify/middie versions 9.3.1 and earlier do not register inherited middleware directly on child plugin engine instances. When a Fastify application registers authentication middleware in a parent scope and then registers child plugins with @fastify/middie, the child scope does not inherit the parent middleware. This allows unauthenticated requests to reach routes defined in child plugin scopes, bypassing authentication and authorization checks. Upgrade to @fastify/middie 9.3.2 to fix this issue. There are no workarounds.

Weakness: CWE-436 (Interpretation Conflict)

## Which versions are affected by CVE-2026-6270?

2 open source packages ship an affected version, including @fastify/middie. It affects 1 product.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `@fastify/middie` | npm | All versions | 9.3.2 |
| `@fastify/middie` | npm | All versions | 9.3.2 |

Products: Fastify Fastify\/Middie.

## How do I fix CVE-2026-6270?

Upgrade @fastify/middie to 9.3.2 or later.

## Is CVE-2026-6270 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6270 exists.

## Other vulnerabilities in @fastify/middie

- [CVE-2026-85184](https://insights.sbomify.com/CVE-2026-85184/): @fastify/middie vulnerable to path-scoped middleware bypass via absolute-form request target (critical 9.1)
- [CVE-2026-14198](https://insights.sbomify.com/CVE-2026-14198/): @fastify/middie vulnerable to authorization bypass via encoded slash in path parameter values (critical 9.1)
- [CVE-2026-22031](https://insights.sbomify.com/CVE-2026-22031/): Fastify Middie Middleware Path Bypass (high 8.4)
- [CVE-2026-2880](https://insights.sbomify.com/CVE-2026-2880/): @fastify/middie has an improper path normalization vulnerability (high 8.2)
- [CVE-2026-14181](https://insights.sbomify.com/CVE-2026-14181/): @fastify/middie standalone engine vulnerable to Denial of Service via malformed percent-encoded paths (high 7.5)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-6270 and fix it if it is affected.

What it is: CVE-2026-6270 is a critical authentication bypass vulnerability in @fastify/middie, published on 16 Apr 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-6270 exists.

Affected packages and fixed versions:
- @fastify/middie (npm): affected all versions; fixed in 9.3.2
- @fastify/middie (npm): affected all versions; fixed in 9.3.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-6270/ (Markdown: https://insights.sbomify.com/CVE-2026-6270/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-6270
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-6270
- OSV: https://osv.dev/vulnerability/CVE-2026-6270

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-6270/
Summaries are generated and may contain mistakes; the official description above is the record.